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<br /> <br />Floodplain <br /> <br />Channel <br /> <br />Development occurs in <br />urban, suburban, small <br />town, and rural settings, <br />and buildings in many of <br />these areas have been lo- <br />cated on hazardous <br />flood-prone sites. <br /> <br />16 <br /> <br />Floodplain <br /> <br />Development in the water- <br />shed alters natural drain- <br />age characteristics and de- <br />creases the permeability of <br />soil, thus exacerbating the <br />effects of flooding. <br /> <br />Riverine Development <br />The introduction of built eiements into flood-prone areas <br />adds a range of factors, other than physiographic, that af- <br />fect the degree of flooding In riverine watersheds, devel- <br />opment has consistently altered the natural topograph~ <br />thus modifying drainage patterns and increasing storm <br />water runoff Development also displaces much of the nat- <br />ural vegetation that would otherwise absorb water, and <br />decreases the permeability of soil by covering it with <br />buildings or with nonporous surfaces (eg, roads, side- <br />walks, and parking} <br />Vltirnately, these development characteristics, either <br />singly or in combination, cause a drastic increase in the <br />amount and velocity of water runoff. Thus, during times <br />of severe storms or heavy snowmelt the river system is <br />quickly filled beyond its capacity and flooding results. <br />The levels of flooding are increased not only by the <br />abnormal runof~ but also because buildings located on <br />the floodplain displace a volume of flood 'water The more <br />buildings on the floodplain, the greater is the displace- <br />ment and the greater the level of flooding. <br /> <br /> <br />0-20% Runoff <br />80--100% Infiltration <br /> <br />40-50% Runoff <br />50-60% Infiltration <br /> <br />90-100% Runoff <br />0-10% Infiltration <br />